1. Field of the Invention
The present invention relates to a variable valve operation timing control device (VTT) for varying the valve operation timing of an internal combustion engine in accordance with the operating conditions of the engine.
2. Description of the Prior Art
As a mechanism for controlling the operation timings of intake valves variably in accordance with the operating conditions of an internal combustion engine, there has been known the variable valve operating timing control device for varying the rotational phase of a camshaft relative to a cam pulley rotating in synchronism with a crankshaft.
The variable valve operation timing control device, as disclosed in Japanese Patent Laid-Open No. 134010/1989, for example, is constructed to include: timing varying means having a hydraulic servo valve for varying the operation (or valve) timings of the intake valves of the internal combustion engine; and hydraulic drive means having a hydraulic cylinder for driving the spool of the servo valve.
In this prior art, moreover, two control valves of the hydraulic drive means are opened or closed to move the servo valve at a constant speed thereby to vary the valve timing of the timing varying means.
Thus in the prior art, the variable valve operation timing control device is complicated because its timing changing means and hydraulic drive means have to be equipped with the separate hydraulic system.
In the prior art, moreover, what is controlled by the two ON/OFF valves is to interrupt the individual supplies of the oil pressures. As a result, the prior art has found it difficult to control the movements of the servo valve delicately and its moving speed and to control the valve operation timing only finely and the moving speed to a desired value. Thus, the prior art is accompanied by a problem that it cannot perform finer controls.
It is, therefore, conceivable to adopt not the ON/OFF valve but a valve capable of controlling the quantity of oil by controlling its degree of opening to change the valve operation timing at a desired rate. If, however, this valve is used, the drive signal of the valve and the change of the valve operation timing may fail to be accurately timed, because of the dispersion in the valve manufacture or the oil leakage from the valve.
This results in another problem that the valve operation timing cannot be held as it is or cannot be varied.